ENANTIOSELECTIVE SYNTHESIS OF BETA-AMINO ACIDS .2. PREPARATION OF THE LIKE STEREOISOMERS OF 2-METHYL-3-AMINOBUTANOIC ACID AND 2-BENZYL-3-AMINOBUTANOIC ACID

An improved procedure for the preparation of (R)- and (S)-3-aminobutanoic acids (2a) through diastereomer separation of the corresponding (1'S)-N-phenethyl derivatives 1 is reported. From 2a, the four possible stereisomeric perhydropyrimidin-4-ones 4 were prepared through the amide 2c and the Schiff base 3. In the cyclization of 3, the cis products 4 predominate ca. 95.5. These heterocycles can be alkylated (LDA, RX), as demonstrated by methylation and benzylation, with formation of a single diastereoisomer (5, 6). Hydrolysis (6 N aqueous HCl) of these 5,6-dialkylperhydropyrimidin-4-ones leads to the free amino acids 7-10.